Running rough on cold starts

[The shuggs]

I've been chasing a timing problem on my Jeep since June. Fortunately my job sends me out of town often so I don't need it much, but it is a pain in the ass on the days I am in town.

The vehicle:
- 1998 SE 2.5L auto.
- 366,000 km

The problem:
- The engine runs rough after starting up until it has warmed up. There's no power under acceleration, and it will buck when given gas. As the temperature climbs, it runs better, but I can't drive it a few minutes after reaching full temperature.
- Initially it only happened on cold mornings, and would be worse the colder it was. Now it happens on every cold start.
- Only once did I since a sputter while driving after it's warmed up.
- The first time it threw a code, it was for the TPS (can't remember the number, but the code went away after replacing the TPS).
- Throws a p1391 code.

Not the problem:
- Starts just fine.
- Once it starts to run well, it will run fine if it's sat for a short time (couple hours in temps around freezing). It has to more or less completely cool down for the problem to come back.
- CMP & CKP Sensor signals are consistent (see pictures), only vary as the computer tries to adjust the timing.

- Distributor,there's no play in shaft and clocking can't be changed.
- I plugged in my scan tool, and CTS, IAT, & MAP sensors all show expected readings. MAP will jump ~1mmHg due to the occasional misfire.

What I've tried:
- New TPS: Mopar, removed the code, but not the problem.
- New IAT: no change.
- New CMP: Mopar, no change
- New cap, rotor, wires, and plugs: no change, were replaced at the same time as the CPS.
- New CKP: Mopar, no change.
- New timing chain, gears, and tensioner: ~12 degrees of slack before replacement, ~5 afterwards. problem went away for ~3 weeks/500km. Came back as temperatures dropped in September.

What I'm thinking:
- Ignition coil might be dying slowly, but they usually get worse as they get warmer.
- ECU might be going.
- because it's related to the engine temperature, it's more likely to be electrical than mechanical.
- Or it's related to the ECU programming.
- I need a second Jeep.

 

[Jerry Bransford]

What is your temperature indicator showing in the instrument cluster when the engine is first started in the morning? You replaced your IAT but did you replace the coolant temperature sender screwed into the thermostat housing? Or is it plugged in and the wiring leading to it is in good condition? It sounds like the computer might think the engine is warmed up and is having the O2 sensors set the air-fuel mixture when the engine is actually cold.

 

[The shuggs]

It shows that it's cold (below 40). I did consider that, so I plugged in my scanner and checked the live data before and after starting. The value corresponded with the dial, and what I would expect. Is it possible if the sensor is just a little off from what it should be?

 

[RaymondT]

I wouldn't rule out the clockspring. See my last post in this thread.

https://wranglertjforum.com/threads...n-cause-engine-to-run-lean.27453/#post-443230

Before I swapped out the clockspring, my 2.5 would only start after 2-3 seconds cranking and stumble up to idle speed (800ish) over another 2-3 seconds. After the clockspring change, it would fire right up in one second, jump up to 1300rpm and settle back down to a smooth idle after 2-3 seconds. Even when it's 40F out! And after a week or so of ECM learning, it's never run so good.

 

[The shuggs]

I wouldn't rule out the clockspring. See my last post in this thread.

https://wranglertjforum.com/threads...n-cause-engine-to-run-lean.27453/#post-443230

Before I swapped out the clockspring, my 2.5 would only start after 2-3 seconds cranking and stumble up to idle speed (800ish) over another 2-3 seconds. After the clockspring change, it would fire right up in one second, jump up to 1300rpm and settle back down to a smooth idle after 2-3 seconds. Even when it's 40F out! And after a week or so of ECM learning, it's never run so good.

The TPS code went away after replacing it with a Mopar sensor, but nothing else changed. That was a couple months ago too. But I will test the clockspring to make sure. Do you or @Jerry Bransford know the best way to test? It's not a cheap part, so I don't want to replace it without a thorough test beforehand.

 

[liquidfluidity]

On board with this. When cold, mine will shoot up , start coming down and them stumble around 1000 rpms for a couple seconds before it evens out.

 

[4Jah]

I'm having the same issue, and not sure what the problem is.. This morning it was only 50F, so not even that cold.. It starts fine, but I have to rev the engine for several minutes to keep the RPMs up long enough to warm up the engine or else the idle drops low enough to kill the engine. Once it's warmed up, it runs fine. Where should I start in diagnosing the issue? I'd like to start easy/cheap to rule things out... thoughts?

FYI - '99 Sahara 4.0L

 

[The shuggs]

I'm having the same issue, and not sure what the problem is.. This morning it was only 50F, so not even that cold.. It starts fine, but I have to rev the engine for several minutes to keep the RPMs up long enough to warm up the engine or else the idle drops low enough to kill the engine. Once it's warmed up, it runs fine. Where should I start in diagnosing the issue? I'd like to start easy/cheap to rule things out... thoughts?

FYI - '99 Sahara 4.0L

I think in your case cleaning your IAC, and throttle body would be the best first step.

 

[4Jah]

I think in your case cleaning your IAC, and throttle body would be the best first step.

Thanks!

 

[The shuggs]

Nobody updates their threads, so I guess I'm nobody.

I gave the engine a thorough cleaning, as there was a long-neglected oil leak that I thought might be messing with a ground connection. On the initial start-up to dry the engine, it did run rough as usual, but on subsequent drying stages it ran fine. This was expected as it was par for the course, as in the past it wouldn't run rough again until it had cooled down completely.

But the next day it started, and warmed up fine. And has still been working well since then, even on cooler, rainy days. The real test will be in the fall as we start to dip below 0. Granted, I'll likely have to keep the engine clean until then, and maybe even fix the leaking gasket.

My cleaning procedure was:
1) Add fuel, and fuel detergent
2) Cover intake, alternator, and sensitive electronics
3) Spray a heavy duty gelling engine degreaser
4) Let it soak for 15 minutes
5) Brush heavily gunked spots
6) Rinse with water
7) Idle for 15 minutes to dry
8) Repeat steps 3-7 with a normal duty degreaser
9) Repeat steps 3-7 with simple green
10) Used a can of Seafoam through the throttle body.

I'll continue to drive it here and there to ensure the problem is gone, then I'll use a can of oil cleaner 50 km before by next oil change to help get out anything that got freed up during the clean.

I suspect that the leak was letting oil seep into the ground connection, interfering with it. But once the engine got warm enough, the oil would thin enough that it could easily flow out of there. But once I shut it down and let it cool down, the oil would thicken enough to mess with the ground signal on start-up.

 

[The shuggs]

The problem is still persisting. One thing I forgot to mention is that when it's running exceptionally rough the speedometer dial will occasionally act all wonky. It was on the warmer side last night, but I was able to record it.

 

[Tkaupelis]

The problem is still persisting. One thing I forgot to mention is that when it's running exceptionally rough the speedometer dial will occasionally act all wonky. It was on the warmer side last night, but I was able to record it.

This happened to me and it was the alternator